It is known in the prior art that an exposure apparatus carries out the exposure process of illuminating the various patterns on the reticles using the exposure light beams, and transferring the patterns onto the substrates such as wafers, glass substrates, etc. coated with photoresist.
In resent years, semiconductor components become more and more integrated, which requires the circuit pattern to be more precise. As feature size decreases, especially with the use of off-axis illumination and phase-shifted mask, the aberrations are having more influence on the quality of lithography process. Thereby, the in-situ measurement technique for optical imaging system in lithography tools is becoming more and more important.
The process of aberration measurement in an optical imaging system will be described as follows with an example of projection optics. Firstly, position the reticle for aberration measurement on the object plane, and image the pattern formed by the reticle onto the substrate via the projection optics. Then, develop the image on the substrate, and use the Scanning Electron Microscope (SEM) to perform the measurement on the developed image. Finally, calculate the aberrations of the projection optics according to the measurement results. (Prior art 1, Peter Dirksen, Casper A. Juffermans, Ruud J. Pellens, Mireille Maenhoudt, Peter Debisschop. “Novel aberration monitor for optical lithography.” Proc. SPIE 1999, 3679, 77-86.)
However, in the abovementioned method, the measurement accuracy is related to the lithographic process, such as coating uniformity and developing uniformity. Moreover, pre-processes such as develop must be carried out before utilizing SEM to measure, which prolongs the process of aberration measurement.
To avoid these problems, a method of wavefront aberration measurement for projection optics is provided by using the Transmission Image Sensor (TIS), i.e. the TAMIS technique. (Prior art 2, Van der Laan, Hans, Dierichs, Marcel, van Greevenbroek, Henk, McCoo, Elaine, Stoffels, Fred, Pongers, Richard, Willekers, Rob. “Aerial image measurement methods for fast aberration set-up and illumination pupil verification.” Proc. SPIE 2001, 4346, 394-407.)
The TIS comprises a set of sub-micron isolated spaces and a square hole, both of which have a photodiode close behind them. Wherein, the isolated spaces comprise X-direction isolated spaces and Y-direction isolated spaces. Isolated spaces of different directions are used to detect the image positions of the spaces of different directions. The square hole is used to compensate the intensity fluctuations of the light source. The image position (X, Zx) of the X-direction spaces and the image position (Y, Zy) of the Y-direction spaces are determined according to the output of the photodiodes.
During the measurement, the binary optical reticle for aberration measurement is positioned on the reticle stage, which has marks with similar pattern to the TIS. The TIS scans the aerial image of the reticle mark projected by the projection optics horizontally and vertically by moving the wafer stage, and obtains the image position (X, Y, Zx, Zy) of the mark. Comparing the image position with the ideal position, the displacement (ΔX, ΔY, ΔZx, ΔZy) of the image is obtained. The image positions of all the marks under different NA and σ are measured to get the imaging displacements at different positions in the field of view under different illumination conditions. Then, the corresponding Zernike coefficients of the aberrations can be calculated by employing the TAMIS algorithm. However higher accuracy of aberration measurement is required to meet the development of lithograph technology. So, it has been more and more difficult for the abovementioned measurement techniques to achieve the high accuracy required.